1. Field of the Invention
The present invention relates to strain gage load cells for weighing, and specifically to a new design for double bending beam load cells, also known as xe2x80x9cSingle Point Load Cellsxe2x80x9d.
2. Description of the Related Art
Known double bending beam load cells are made from blocks of special load cell quality aluminum or steel alloy with a cut-out in the middle forming parallel top and bottom bending beams connected by solid end blocks. One of the end blocks is bolted to a base so the bending beams extend from the base as cantilevers, and the second block is adapted to receive a load that will cause the two bending beams to bend, each forming an S-shaped curve. Strain gages bonded to the bending beams on each side of their inflection points are connected in a strain gage bridge used to sense the load on the load cell. Such double bending beam load cells have been used extensively in the art for several decades, and are widely described in the patent literature. See for example U.S. Pat. Nos. 2,866,059 and 3,512,595.
The load cell material for high accuracy load cells is expensive, and the machining of the central cut-out is costly, so attempts have been made to lower the cost of double bending beam load cells by bolting separate bending beams to end blocks of less costly metal. Such devices are described in U.S. Pat. No. 5,076,376 issued to Bizet et al. These prior art load cells are inexpensive, but the bolted joints rely solely on friction between bending beams and end blocks, so the joints are subject to slip under shear stress. Shear stress between end blocks and beams is what forces a double bending beam load cell to deform as a parallelogram under load, with the end blocks remaining vertical and the beams flexing in parallel S-shapes. Any slip in these joints will change the load cell geometry and cause measurement errors. These prior art double bending beam load cells accordingly suffer from hysteresis, creep, and lack of repeatability, so they have not found acceptance in the high precision load cell market.
An object of the present invention is to provide a high accuracy double bending beam load cell that is inexpensive to make.
Another object is to provide an inexpensive double bending beam load cell that does not suffer from significant creep or hysteresis.
These objects are obtained by a double bending beam load cell comprising two parallel bending beams of load cell quality material, at least two separate end blocks of less expensive material to maintain a fixed spacing between said bending beams, and slip free joints to join said end blocks and said bending beams.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.